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--- doomsday [2023-04-19 05:27] – created root
+++ doomsday [2023-04-20 12:06] (current) – root
@@ Line 5: / Line 5: @@
 It's a nifty parlor trick, a decent mental exercise, and takes about as much skill as learning to solve a Rubik's cube in under a minute.
-==== Monthly doomsdays ====
+===== Monthly doomsdays =====
 The monthly doomsdays (after February) are trivial to remember: they fall on the same day as the month's number.
@@ Line 54: / Line 54: @@
 | September, December | 5, 12, 19, 26 |
 | June | 6, 13, 20, 27 |
+Can you figure out this year's doomsday if you know the current date and day of the week? Try calculating other dates this year and consult a calendar to see whether it works out.
+===== Yearly doomsdays =====
+Given the last exercise, with enough practice, calculating days of the week should be fairly trivial after you learn a year's doomsday. But what about doomsdays for other years? A simple algorithm was developed for this. The century doomsday of this century is **Tuesday** . This algorithm is based around the decade and digit years, eg for dates 2000 - 2099:
+  - Is the current 2 digit year odd? If so, add 11.
+  - Divide it by 2.
+  - Is the current 2 digit year odd? If so, add 11.
+  - Count how many days it is from your new number to a multiple of 7.
+In case you're rusty on your 7s tables, the multiples of 7 are: 0, 7, 14, 21, 28, 35, 42, 49, 56, 63, 70, 77, 84, 91, 98, 105. Let's look at 2 examples of this algorithm in practice:
+:
+  - is odd, add 11. -> 34
+  - divide by 2 -> 17
+  - is odd, add 11. -> 28
+  - 0 days until a multiple of 7 (28)
+That means 2023 has the same doomsday as the century year, Tuesday. Let's try another:
+:
+  - isn't odd -> 36
+  - divide by 2 -> 18
+  - isn't odd -> 18
+  - 3 days until a multiple of 7 (21)
+Tuesday + 3 = Friday. Lo and behold, Friday is the doomsday of 2036.
+{{tag>math wip}}